Southern California -- this just in

Authorities were continuing their investigation Thursday into the cause of an explosion that killed one person and injured three others in a South Los Angeles building.

The blast tore through a business by a small market in the 2500 block of South Grand Avenue just before 6:30 p.m. Wednesday, Los Angeles Fire Department officials said. Four people were injured in the blast -- two critically -- and one of the victims later died, they said.

Authorities have not released the victim's name or the names of those injured in the blast.

The business, which had no apparent sign and was tucked behind the market fronting Grand Avenue, stores nitrous oxide cylinders used to power mini-bikes and go-carts, according to a law enforcement source familiar with the investigation.

Investigators were unsure what caused the explosion but were probing whether a worker was filling one of the cylinders when it blew up, according to the source. LAFD Deputy Chief Mario Rueda said firefighters found cylinders in the establishment, but declined to say what caused the explosion.

The victims, who were found inside the building, were identified as two adult males and two adult females. Two were in fair condition, the LAFD said. Rescuers carried out a victim covered in blood who appeared to have severed limbs, a neighbor at the scene said.